1. Field of the Invention
This invention relates generally to magnetic recording disk drives, and more particularly to a system and method for measuring readback signal amplitude asymmetry in a perpendicular magnetic recording disk drive.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in the generally planar recording layer in a generally perpendicular or out-of-plane orientation (i.e., other than parallel to the surfaces of the disk substrate and the recording layer), is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. A common type of perpendicular magnetic recording disk drive uses a “dual-layer” disk. This type of disk drive is shown schematically in FIG. 1. Write current passes through a coil of the write head to generate a magnetic field at the write pole. The dual-layer disk includes a perpendicular magnetic data recording layer on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL) formed on the disk substrate. The SUL serves as a flux return path for the magnetic field from the write pole to the return pole of the write head. The recording layer has perpendicularly recorded magnetizations or magnetized regions that form a data track, with adjacent regions in the data track having opposite magnetization directions, as represented by the arrows. A sense current passes through the read head, typically a magnetoresistive (MR) read head, such as a tunneling MR (TMR) read head in which sense current passes perpendicularly through the layers making up the head. The magnetic transitions between adjacent oppositely-directed magnetized regions cause changes in electrical resistance that are detectable by the read head as data bits. A shield of magnetically permeable material prevents fields from magnetizations other than the magnetization being read from reaching the read head.
The read head and write head are typically formed as an integrated read/write head on an air-bearing slider. The slider is attached to an actuator arm by a suspension and positioned very close to the disk surface by the suspension. The actuator moves the slider across the disk surface so that the read/write head can access the data tracks. There are typically a stack of disks in the disk drive with a slider-suspension assembly associated with each disk surface in the stack.
In a perpendicular magnetic recording disk drive the amplitude of the readback signal from the read head is asymmetric as a natural result of the construction of the read head. It is believed that stray magnetic fields arising from the media background may also contribute to amplitude asymmetry. Readback signal amplitude asymmetry means that the amplitudes of the pulses from magnetizations recorded in one direction (e.g., the “positive” direction) are different from the amplitudes of the pulses from magnetizations recorded in the opposite direction (e.g., the “negative” direction). Thus the amplitude asymmetry (AASY) measured in percent can be expressed by the following equation:AASY=[(POS−NEG)/(POS+NEG)]*100,  Equation (1)where POS represents the measured amplitude of the pulses recorded in one direction and NEG represents the measured amplitude of the pulses recorded in the other direction.
A high value of AASY is undesirable because it is correlated with a high bit error rate (BER) when the data is read back. Thus it is important to be able to accurately measure AASY to both improve the design of the read channel to improve the BER and to determine which heads to accept for use during disk drive manufacturing.
The conventional approach for AASY measurement is with the use of a head-disk tester (also called a spin stand). Most head-disk testers include a spectrum analyzer that is used for testing many read and write head parameters, such as spectral signal-to-noise ratio (SNR) for the read head and overwrite (OW) for the write head. To measure AASY a special pattern is written on the disk to generate a readback signal of alternating isolated positive and negative pulses. The pattern is then read back by the read head. The readback signal is illustrated in FIG. 2, which shows the series of alternating isolated positive and negative pulses. The “noisy” baseline regions between the pulses that isolate the pulses is the readback signal from a high-frequency pattern of alternating positive and negative magnetizations written on the disk. However, the spectrum analyzer in the head-disk tester can not measure the amplitudes of these isolated alternating positive and negative pulses. Thus a special peak detection channel is required in the tester to measure the amplitudes of both the positive and negative readback pulses in the time domain. This peak detection channel is also used to measure other read head parameters relating to signal amplitude, such as middle-frequency track average-amplitude (MFTAA) and low-frequency track average-amplitude (LFTAA). The AASY is then calculated, according to Equation (1), from the measured amplitudes. The need for a peak detection channel increases the cost of the head-disk tester. Also, because every head must be tested, this cost is amplified because a large number of testers are required to handle the high volume of head production. This AASY measurement method also increases the overall test time for each head because the measurement with the peak detection channel must be done separately and in addition to the other measurements performed with the spectrum analyzer.
What is needed is a head-disk tester and method for accurate AASY measurement that does not require a peak detection channel and that does not require additional test time.